Colorado River Delta

Colorado River Delta as seen from space (2004); Isla Montague is the large island in the center.

The Colorado River Delta is the region where the Colorado River flows into the Gulf of California (also known as the Sea of Cortez) in eastern Mexicali Municipality in the north of the state of Baja California in northwestern-most Mexico. The delta is part of a larger geologic region called the Salton Trough.[2] Historically, the interaction of the river's flow and the ocean's tide created a dynamic environment, supporting freshwater, brackish, and saltwater species. Within the delta region, the river split into multiple braided channels and formed a complex estuary and terrestrial ecosystems. The use of water upstream and the accompanying reduction of freshwater flow has resulted in the loss of most of the wetlands of the area, as well as drastic changes to the aquatic ecosystems. However, a scheme is currently in place which aims to rejuvenate the wetlands by releasing a pulse of water down the river delta.[3]

Contents

Natural history

Until the early 20th century, the Colorado River ran free from its headwaters in the Rocky Mountains of Colorado southwest into Mexico, where it flowed into the Gulf of California. Significant quantities of nourishing silt from throughout the Colorado River Basin were carried downstream, creating the vast Colorado River Delta.

Prior to the construction of major dams along its route, the Colorado River fed one of the largest desertestuaries in the world. Spread across the northernmost end of the Gulf of California, the Colorado River delta's vast riparian, freshwater, brackish, and tidalwetlands once covered 7,810 km² (1,930,000 acres) and supported a large population of plant, bird, aquatic, and terrestrial life. Because most of the river's flow reached the delta at that time, its freshwater, silt, and nutrients helped create and sustain a complex system of estuarial wetlands that provided feeding and nesting grounds for birds, and spawning habitat for fish and marine mammals. In contrast to the surrounding Sonoran Desert, the Colorado River delta's abundance was striking.

On the map the Delta was bisected by the river, but in fact the river was nowhere and everywhere, for he could not decide which of a hundred green lagoons offered the most pleasant and least speedy path to the Gulf.

After dam construction

Today, conditions in the delta have changed. Like other desert river deltas, such as the Nile Delta and the Indus River Delta, the Colorado River delta has been greatly altered by human activity. Decades of dam construction and water diversions in the United States and Mexico have reduced the delta to a remnant system of small wetlands and brackish mudflats.[4] As reservoirs filled behind dams and captured floodwaters, freshwater could no longer reach the delta.

The construction of Hoover Dam in the 1930s marked the beginning of the modern era for the Colorado River Delta. For six years, as Lake Mead filled behind the dam, virtually no freshwater reached the delta. Even spring flooding was captured. This ecologically devastating event was repeated from 1963 to 1981 as Lake Powell filled behind the Glen Canyon Dam. With these reservoirs now filled, the dams are used to regulate flow so that water can be reliably apportioned among the users of the Colorado River Compact, and its use maximized. Most flood flows can be contained, regulated, and added to the river's capacity to sustain the Western United States' urban centers and agriculture. Floodwaters are released only when the Bureau of Reclamation, the agency managing the dams, predicts flows that exceed the system's capacity for use and storage.

The loss of freshwater flows to the delta over the twentieth century has reduced delta wetlands to about 5 percent of their original extent, and non-native species have compromised the ecological health of much of what remains. Stress on ecosystems has allowed invasive plants to out-compete native species along Colorado River riparian areas. Native forests of cottonwood and willow have yielded to sand and mudflats dominated by the nonnative tamarisk (also known as salt cedar), arrow weed, and iodine bush, a transformation that has decreased the habitat value of the riparian forest (Briggs and Cornelius, 1997).

High flows in 1980s

Full reservoir conditions coupled with a series of flood events throughout the 1980s and early 1990s resulted in flood releases that reached the delta. These flows reestablished an active floodplain and revegetated many areas of the floodplain within irrigation and flood control levels, and helped to reestablish riparian forests.

Ecology

The delta supports a variety of wildlife, including several threatened and endangered species. Mexico's Environmental Regulations on Endangered Species lists the following endangered species found in the terrestrial and aquatic regions of the delta (Diario Officiel, 1994):

Although not extensively studied, the delta's significance for migratory birds is indisputable, as it is the principal freshwater marsh in the region. A total of 358 bird species have been documented in the Colorado River Delta and the upper Gulf of California region. From these, two are listed as endangered, six as threatened, and sixteen are under special protection in Mexico. Two wintering species and five breeding species have been locally extirpated, including the southwestern willow flycatcher, the Fulvous whistling duck, and the sandhill crane.

Biosphere reserve

The Gulf of California lies within the jurisdictional boundaries of Mexico and its states of Baja California and Sonora. In 1974, the Mexican government designated portions of the upper Gulf and lower Colorado River Delta as a reserve zone.

The United Nations Educational, Scientific and Cultural Organisation (UNESCO) designated over 12,000 km² (3 million acres) of Gulf of California and Colorado River Delta Biosphere Reserve as a BiosphereNature reserve in June 1993. Within this 12,000 km² (3 million acres), over 4,000 km² (1 million acres) nearest the Colorado River Delta are designated as the Reserve "core area", with the remaining 8,000 km² (2 million acres) of open water and shoreline designated as a "buffer area".[6]

UNESCO considers areas for designation as Biosphere Reserves only after the nation in which the site is located submits a nomination. Once designated, Biosphere Reserves remain under the sovereign jurisdiction of the countries where they are located. Federal Mexican governmental agencies with administrative authority over the Biosphere Reserve include the National Commission of Protected Natural Areas (CONANP) and the Secretary of the Environment, Natural Resources and Fisheries (SEMARNAP).

In addition to designation as a Biosphere Reserve, 2500 km² (618,000 acres) within Colorado River Delta (Humedales del Delta del Río Colorado) are designated as a Ramsar Wetland under the U.N. Convention on Wetlands. Ramsar Wetlands are wetlands of international importance in terms of their ecology, botany, zoology, limnology or hydrology. The U.N. designation is considered following a nomination by the nation in which the site is located.